Xanthan gum is produced commercially by batch fermentation in submerged culture. Xanthomonas juglandis and X. campestris are examples of suitable organisms. Culture fluids develop extremely high viscosity and pseudoplasticity, which have a seriously detrimental affect on the oxygen transfer capabilities of conventionally designed and operated fermenters. This inevitably leads to the fermentation becoming limited by the oxygen transfer rate supported by the reactor and results in low xanthan yields and extended fermentation times.
The above difficulties could, in theory, be overcome in two different ways:
(i) by improvements in fermenter design, resulting in higher aeration efficiencies.
(ii) by controlling the oxygen demand of the culture without affecting productivity.
This invention is concerned with the latter approach. Polysaccharide biosynthesis is an energy-consuming process and oxygen is therefore required during fermentation for both growth and xanthan biosynthesis, which occur simultaneously in the conventional batch processes. The oxygen requirements for the two processes may be estimated as follows:
(i) Oxygen required for cellular biosynthesis
When a typical prokaryotic microorganism grows aerobically on a medium in which glucose is the sole carbon source, approximately 50% of the carbon is catabolised to produce carbon dioxide and water with the generation of ATP. ##STR1## The remaining carbon is metabolised to produce cellular material. Thus for every mole of glucose utilised for cellular biosynthesis, 3 moles of diatomic oxygen are required. The figure of 16 moles of ATP per mole of glucose is a generally accepted figure for prokaryotic microorganisms.
(ii) Oxgen required for xanthan biosynthesis
The biosynthetic pathway of xanthan gum is not fully understood but it is generally considered that the addition of a single hexose monomer unit to the xanthan polymer requires two molecules of ATP. ##STR2## Thus, for every 9 moles of glucose utilised for xanthan biosynthesis, 6 moles of diatomic oxygen are required.
From these estimates it can be deduced that cellular biosynthesis requires 4.5 times more oxygen than xanthan biosynthesis.
The objective of the present invention is to regulate the oxygen demand by the separation of growth and product biosynthesis using a two-stage fermentation process. In the first stage, conditions are employed that permit growth of the organism but not xanthan biosynthesis. Since a high viscosity will not develop in the absence of polysaccharide, the relatively high oxygen demand, similar to that experienced to conventional fermentations, can be met without problems. In the second stage, growth will be prevented and only polysaccharide biosynthesis will occur. Although a high viscosity will inevitably develop, the oxygen demand of the culture will be significantly lower than in conventional processes since energy will not be required for growth but only xanthan biosynthesis. The oxygen requirement of the organism will thus be more easily satisfied. Furthermore, by controlling the amount of growth occurring in the first stage, the oxygen demand during the second stage may actually be regulated.